This invention relates generally to optical techniques of endpoint detection, particularly those used for monitoring and controlling processes of manufacturing integrated circuits on semiconductor wafers and flat panel displays on large area substrates.
Several non-contact, optical techniques for detecting the endpoint of various semiconductor processing steps are presently being used or have been suggested. Many of these techniques involve directing electromagnetic radiation in the visible or near visible portion of the spectrum from a source onto the surface being monitored, with some of the reflected radiation being captured by a photodetector. An electrical signal output of the photodetector is analyzed while processing of the substrate surface is taking place.
One such technique monitors the development of a photoresist masking layer which has previously been exposed to a light pattern that defines the regions of the layer desired to be totally removed and those desired to remain and act as a mask. A developer solution is applied to the exposed photoresist layer in order to remove material in the desired regions. As material is being removed, the thickness of the layer in those regions gradually decreases. A detectable component of light that is reflected onto the photodetector cycles in intensity between a maximum and a minimum as a result of interference of light from the source reflected from the top of the layer and that reflected from the bottom, since the photoresist material is substantially transparent. When the material in these areas has been removed in order to first expose the surface below, a condition called "breakthrough", the varying optical signal at the photodetector terminates. An electronic processing system receiving the photodetector output signal determines when this occurs for the purpose of either notifying the operator concerning the endpoint condition or automatically terminating the development process at endpoint. A similar monitoring technique is used in a process of etching through a patterned layer of a substantially transparent dielectric material, such as an oxide.
Conversely, a similar technique has also been suggested for monitoring the formation of a layer by counting the number of times that the photodetector output signal cycles between its maximum and minimum values. The number of cycles that has occurred at any particular time is proportional to the thickness of the layer being monitored at that time. When a number of cycles of intensity variation has been counted that corresponds to a desired layer of thickness, the detection of this endpoint condition can cause the formation of the layer to terminate.
A similar light reflective technique is used in the selective removal of an opaque layer of material that has a reflectivity different from that of a layer below. As the material is being removed, by an etching process or otherwise, the reflected signal remains substantially the same until a breakthrough occurs, at which time the signal level changes either up or down depending upon whether the layer below is more or less reflective, respectively, than the layer being processed.
Although the major part of the discussion herein is directed toward applications in processing integrated circuits on semiconductor wafers and forming flat panel displays on large area substrates, the monitoring techniques being described also find application in various other specific fields. For example, the light reflection technique can monitor the progress of any selective or total removal of a layer of material from a substrate.
It is a primary object of the present invention to provide an improved technique for monitoring such processes by a light reflective technique with improved reliability and accuracy.
It is another object of the present invention to provide a source of the reflected light and the photodetector in a single sensor housing that is small, rugged and well suited for hostile industrial processing applications.